Undulating beam traveling wave tube focusing structure



4 Sheets-Sheet 1 R. M. PHILLIPS June 8, 1965 UNDULATING BEAM TRAVELING WAVE TUBE FQCUSING STRUCTURE. Filed Sept. 26, 1961 R. M. PHILLIPS 3,188,518

June 8, 1965 UNDULATING BEAM TRAVELING WAVE TUBE FOCUSING STRUCTURE 4 Sheets-Sheet 2 Filed Sept. 26, 1961 imm gil

3 BY E25 zo! l ATTomvn June 8, 1965 R. M. PHILLIPS 3,188, 518

UNDULATING BEAM TRAVELING WAVE TUBE FOCUSING STRUCTURE Filed Sept. 26, 1961 4 Sheets-Sheet 5 ETE. 5

Foefer/7Bm .1. ms. BY

KJV/5M June 8, 1965 R. M. PHILLIPS 3,188,518

UNDULATING BEAM TRAVELING WAVE TUBE FOCUSING STRUCTURE Filed Sept. 26. 1961 4 Sheets-Sheet 4 ["'iillll ATTORNEY `its low-power handling capabilities.

United States Patent O 3,188,518 UNDULATING BEAM TRAVELING WAVE TUBE FOCUSING STRUCTURE Robert M. Phillips, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed Sept. 26, 1961, Ser. No. 140,875

18 Claims. (Cl. 315-39) This invention relates to apparatus for providing interchange of energy between a stream of charged particles and an electromagnetic wave and more particularly to a system for `focusing the charged particle stream of such an apparatus wherein interchange of energy is effected by forcing said stream to travel with an undulatory motion.

In a traveling wave tube, an electron stream` is exposed to the electric and magnetic fields of a traveling electromagnetic wave over an extended region .along the axis of propagation of the traveling wave. This exposure of the stream is effected by projecting the electron stream along `such axis `for a distance equal to several operating wavelengths of the wave. In a copending application, S.N. 816,540, tiled May 28, 1959, now Patent No. 3,129,356, issued April 14, 1964, by R. M. Phillips and assigned `to `the assignee of the instant inventio-n, a traveling wave` tube is disclosed wherein the electron stream is forced to undulate about the axis .of wave propagation in the region of its exposure to the traveling Wave. The undulating motion is forced on the stream by a static magnetic eld oriented perpendicularly to the axis of Wave propagation and alternating as a function of ldistance along the axis. The undulating stream has an alternating component of velocity transverse to the axis, so that the electrons in the stream will be ac- By adjusting the axial velocity of the stream .to have a Vparticular synchronous relationship ywith the axial phase velocity of the electromagnetic wave, an energy-exchanging interaction between electron stream and traveling electromagnetic wave will occur. Thus, the wave may be amplified by extraction of energy from the stream or the energy of the stream may be increased by extraction of energy from the wave; a travelingwave tube employing the former type of interaction being known `as a traveling wave ampliiier and, employing the latter type, being known as an electron accelerator.

Among the advantages of the above-described invention employing an undulating electron stream is that an energy-exchanging interaction is possible without employing a slow-wave structure, such as a helix or loaded waveguide, to slow the axial velocity of propagation of the wave. Elimination of the slow-wave structure eliminates the accompanying disadvantages, such `disadvan- .tages including costliness :of the slow-wave structure and Accordingly, the aforementioned application discloses non-loaded rectangular, circular` or coaxial waveguides for lguiding the traveling electromagnetic wave along an axis for interaction with an undulating electron stream. The electromagnetic waves may have phase velocities greater than the velocity of light in such non-loaded waveguides.

The electron stream employed in the traveling wave tubes of the above-mentioned application is projected `into` the interaction region with a predetermined cross section, wherein the electrons are closely spaced to obtain a high-etliciency, high-power device. Because each of the electrons has a negative charge, it tends to repel .theyothers `This mutual repulsion of `the electrons tends to enlarge the cross section of, or defocus, the stream.

n In one embodiment of the aforementioned application, the electron stream undulates in a plane oriented i .and therefore is not available for interaction with the electromagnetic wave over the desired axial distance, tends to have lower eiciency `and power output and Afails to make maximum use ofthe initial electron stream provided. Furthermore, such interception of part of the electron stream may cause sufcient heating of the `waveguide walls to require the employment of costly `and complex cooling measures. Thereafter, it is desirable to provide apparatus for employment with a traveling Wave tube of the type described for maintaining the electron stream focused throughout the length of the interaction region to prevent interception of the electron .stream bythe waveguide walls.

j Accordingly, it is the principal object of this invention `to provide apparatus for focusing la stream ofcharged particles. i

vIt is another object of this invention to provide api i `ratus for focusing the'electron stream of a traveling'wave tion, of a portion of the traveling wave tube of FIG. 1;

tube.

yIt is another lobject of this invention to provide apparatus for preventing interception .by the wave-guide of `an undulating .electron stream traveling-along the interior of such waveguide and interacting with an electromagnetic wave traveling therethrough. j

The foregoing objects are achieved by providing, in a traveling wave tube of the type above described, a

static magnetic focusing lfield to impel the loutermost undulating electrons into the electron stream with sufficient force to overcome the defoousing electron repulsion forces. in one form of the instant invention, a static spatially alternating magnetic -iield is employed both to force undulation of .the beam and to provide the desired focusing action. This static magnetic iield is oriented perpendicularly to the waveguide axis .and to the -direction of orientation of an electric field component of the wave and alternates as a function of distance ,along the waveguide axis. Such a magnetic field is provided by a plurality of magnetic pole pieces arranged along the length of the exterior of the waveguide, Where- `in the magnetic polarity of adjacent pole pieces alternates. The spatially alternating magnetic lield forces the electron stream to undulate about the waveguide axis as the stream move-s down the waveguide. Additionally, the magnetic pole pieces are shaped t-o provide 'a stronger concentration of magnetic flux across the waveguide near the extremities of the undulatory displacement of the beam than at the waveguide axis, thereby exerting a net force to impel the outermost electrons `into the stream and prevent defocusing.

The invention will be described with reference to the accompanying drawings, wherein:

'FIGURE 1 is an elevational view, partly in` cross section, of the traveling wave tube of this invention;

FIGURE 2 is a perspective view, partly in cross secaisais FIGURE 3 is a cross-sectional view of one embodiment of the traveling wave tube of FIG. 1;

FIGURE 4 is a cross-sectional view of another embodiment of the traveling wave tube of FIG. 1;

FIGURE 5 is a partial cross-sectional view schematically illustrating the operation of the apparatus of FIG. 1;

FIGURE 6 lis a perspective View, partially in cross section, of a portion of an additional embodiment of the traveling wave tube of FIG. l; and

FIGURE 7 is a cross-sectional View of the embodiment of FIG. 6.

The traveling wave tube of FIGS. 1 and 2 provides an energy-exchanging interaction between an electron stream and a traveling electromagnetic wave in an evacuated waveguide 10. Waveguide Iii is adapted to propagate an electromagnetic wave along the direction of axis 11 of the waveguide in a manner well known in the art. An electron gun 13 is disposed to project an electron stream 14, shown pictorially, along axis 11 for interaction with the wave traveling therealong. An input transition section 16 is coupled to waveguide 10 near one end thereof for launching the wave in the waveguide, transition section 16 receiving input electromagnetic energy through a gas-tight dielectric window 17. An output transition section 19 is coupled to waveguide 1t) near the other l end thereof for receiving the amplified electromagnetic waves from the waveguide, transition section 19 transmitting these waves to utilization apparatus through a gastight dielectric window 211. Windows 17 and 2@ are composed of a material that is transparent to electromagnetic Y waves, but which does not permit the'passage of gaseous molecules, thereby permitting the maintenance of a vacuum in waveguide 1li and transition sections 16 and 19.

Waveguide 1li is of sufficient height to permit undulation of the electron stream about the axis 11, such undulation taking place along a curve lying inthe plane of FIG. 1. Transition sections 16 and 19 are each tapered in height to provide a smooth transition between the height of conventional waveguides, for coupling to transition sections 16 and 19, and the extended height of waveguide Il). A pair of 45 matching ramps 22 and 23 are disposed at opposite ends of waveguide to prevent reflections of the electromagnetic waves in transferring from transition section 16 to waveguide Iii and in transferring from waveguide 10 to transition section 19. Each of ramps 22 and 23 is provided with an aperture therethrough to permit passage of the electron stream.

Electron gun 13, one type of electron gun suitable for use with the instant invention, comprises an indirectly heated cathode 25 and a cathode heater 26. Heater 26 is connected to a suitable energizing source, not shown, for heating the cathode to a temperature to emit electrons. A centrally apertured focusing electrode 2'7 and a correspondingly apertured accelerating anode 28 are provided for projecting the electrons emitted by cathode 2S through the aperture in ramp 22 and along axis 11 of waveguide 10. Electrode 27 and anode 2S also func- -tion -to focus the electrons into a concentrated narrow stream. A suitable potential, not shown, is provided between cathode 25 and anode 2S to project the electron stream along waveguide 10 with the proper velocity for yan energy-exchanging interaction with the electromagnetic netic wave traveling to the right in waveguide 1li of FIGS.

l and 2. As described in the aforementioned patent aplwalls 45.

plication, an energy exchanging interaction between the electron stream and the wave is eiected in waveguide 10 by forcing the electron stream to undulate about the axis of the waveguide. The undulatory motion is forced on the electron stream by a static magnetic lield oriented perpendicularly to the axis of waveguide 1t) and alternating as a function of distance along the length of the waveguide. This static magnetic eld is provided by a plurality of magnetic pole pieces 35 extending along opposite sides of waveguide 1t). The pole pieces along each side of waveguide I@ alternate in magnetic polarity. Thus, along one side of waveguide I0 a north pole piece is disposed between a pair ofsouth pole pieces and a south pole piece is disposed between a pair of north magnetic pole pieces. Each pole piece on one side of waveguide 10 is disposed opposite a pole piece of opposite magnetic polarity on `the other side of the waveguide. Such a pair of opposed poled pieces of opposite polarity maybe termed a pole piece pair. Thus, in a pole piece pair, north and south magnetic pole pieces oppose each other on opposite sides of waveguide 11i.` In this mannenmagnetic ilux is directed across the width of the waveguide from the north magnetic pole piece to thesouth magnetic pole piece of each pole piece pair and this flux alternates in direction along the length of waveguide 11i, providing a spatially alternating static magnetic field.

Magnetic pole pieces 35 may be either permanent magnets or the core members of electromagnets. In the embodiment illustrated in FIG. 2, the pole pieces are chosen to be the core members of electromagnets and, therefore, each pole piece has a solenoid 36 wound therearound. However, for convenience of illustration, only two pole pieces are shown as having solenoids 36 cooperating therewith. Each solenoid 36 is provided with a pair of lead wires 37 to provide electric current for energizing the solenoid. A magnetic return element 38 of soft iron is joined to one end of all p'ole pieces 35 on each side of the waveguide to provide a magnetic return path to minimize the reluctance of the magnetic circuit. Only one such return element 3S is shown, for convenience of illustration.

In accordance with the principles of the instant invention, each magnetic pole piece is provided with a pair of legs 40 extending beyond the face of each pole piece. It is the function of theseV extending legs 40 to form a magnetic iield pattern extending across waveguide 10 that is stronger in intensity away from axis 11 of waveguide 10 than at axis 11. It is this novelly formed magnetic ux pattern across the waveguide that provides the necessary focusing action to prevent electron stream interception by the walls of waveguide 1li. The functions and theory of operation of this invention will be described in further detail hereinafter.

kIn one embodiment of the instant invention, waveguide 10 is a rectangular waveguide, as shown in FIGS. 3 and 4. In this embodiment waveguide 10 comprises a pair of opposite narrow walls 44 and a pair of opposite broad In the coordinate `system illustrated, the broad dimension of the waveguide, the waveguide width, is shown to be parallel to the x-coordinate and the narrow dimension, the waveguide height, is parallel to the ycoordinate. The axial direction, the direction in which the electron stream is projected, is the z-coordinate.

One rectangular waveguide mode which may be propagated along waveguide 10 and which will interact with an electron stream forced in-to undulation by the magnetic structure of FIGS. l and 2 4is a transverse electric wave.

'of the TEM, mode wave is oriented vertically (in the ydirection) and has maximum intensity at the center of the waveguide width and diminishes to zero intensity at the narrow walls 44. The strength of the electric iield pattern :shown in FIG. 4 varies along the axis (zdirection) of the waveguide in a sinusoidal manner, the axial distance between two points of extreme strength and like orienta-` tion being known as a guide wavelength.

i It is shown in the aforementioned patent application' that, if the electron stream, in moving along the waveguide, is `forced toundulate so as to have a periodic velocity component parallel t-o the direction of the electric iield component of the electromagnetic wave, an energyexchanging inter-action between stream and wave may occur. FIGURE 5 illustrates the path of undulation of electron stream lid of FIG. 1 in the region of waveguide .that is included between the alternating magnetic pole pieces 3'5. The solid curved lines in FIG. l5 illustrate the boundaries of an electron stream that remains focused, while undulating, in accordance with the principles of this invention. It is shown in this figure that the electron stream is periodically displaced in the y-direction from waveguide axis 11. This periodic displacement is due to the Ifact that the electron stream in crossing the alternating static magnetic eld is subjected alternately to upward and downward forces. These forces induce in the stream a component of velocity that is perpendicular to the projected velocity in the z-direction. This y-component of velocity is also periodic in nature and is shown in FIG. 5 to be Zero between the pole pieces of each pole piece pair, for example, at positions a, b and c, and to be maximum midway between adjacent pole piece pairs, for example, at positions d and e.

It is known that an electric field exerts a force on a charged particle, `such as yan electron, this force taking place in .the direction of the electric field. If the electron is moving, the electric iield will either increase or retard the velocity of the electron according to the relative di- -rection of the electron and the electric eld. If, therefore, each time that a particular segment of the elect-ron stream has a maximum component of velocity in the ydirection the segment is immersed in a decelerating electric lield, the stream velocity will progressively decrease and the stream will give upenergy to the electric field. Accordingly, .the wave will be amplified as wave and electron stream progress along the length of the waveguide and the device is termed a traveling wave amplifier. If, on the other hand, a particular segment of the electron stream is immersed in an accelerating electric eld each time that the segment has a maximum y-component of velocity, the electron stream will progressively gain energy 'from the wave. Such `a device is known as an electron accelerator. Thus, by adjusting the axial velocity of the electron stream so that the maximum y-component of velocity of a segment of the undulating stream always encounters a decelerating electric iield or always encounters an accelerating electric iield, a synchronous relationship is established and energy is exchanged between stream and wave. Such a synchronous relation takes place when the electron stream progresses through a distance equal to one period of the alternating static magnetic field (distance along the waveguide axis between two like pole piece pairs) during a time lsubstantially equal to that required vfor the electromagnetic Wave to progress through a` distance equal to such static magnetic period plus or minus an integral number of guide wavelengths of the electro magnetic wave. When operating in this synchronous` manner, the entire electron stream is originally in synchronism .with the wave so that some segments have their y-component of velocity increased and other segments have their y-component of velocity decreased. The alternating static magnetic iield periodically converts the entire y-component of stream velocity .to a z-component of velocity. piece has only a z-com-ponent of velocity, the static magnetic iield periodically bending the stream to merge the (In FIG. 5 the electron stream opposite each pole y-component and the z-component.) Thus, the synchronous electron stream contains segments having increased axial velocity and segments having decreased axial velocity. Electron bunches thereby `form inthe stream in a manner well known in prior art traveling wave tubes. With proper relative wave and stream velocities, the undulating bunches a-re formed in decelerating electric iields of the wave land then progressively lose energy to the wave to provide traveling wave amplification.

The static magnetic eld which forces the undulation of the electron beam is that component (the x-compo nent) of the magnetic rield which is directed perpendicularly to lthe plane of the diagram of FIG. 5 (the y-z plane), or perpendicularly to the narrow walls 44 of FIGS. 3 and 4. If lthe face of the pole piece were disposed only adjacent the narrow walls i4 of waveguide 10, this x-.component of the magnetic ield either would be substantially uniform from top to bottom of, the waveguide or would be weaker above and below the axis of the waveguide than at the axis. An electron stream undulating in such a field would tend to become defocused or to spread in the y-direction, as shown by the dotted` lines of FIG. 5, because of the mutual repulsion of the electrons in the stream. This deiocusing could be sufcient, with `an intense and` highly concentrated electron stream, to force a substantial portion of the electrons to be intercepted by the waveguide wall. Interception of the stream by the walls reduces the output power, reduces tube eliiciency, and tends to heat the waveguide walls. By providing the novel pole pieces of the instant nventiom this def-ocusing of the electron stream is avoided and interception of stream electrons by the waveguide walls is eliminated.`

One form of such novel pole piece which has been employed with rectangular waveguide isshown in FIGS. 2, 3, and 4. In these 4ligures, each pole piece face lies adljacent one of the waveguide narrow walls 44. i Fach pole piece is provided with a leg 4t) extending beyond the face and along theb-road walls 45 of thewaveguide. -Legs 40 provide an x-component of .the static magnetic :field that is stronger above and below the waveguide axis than at the axis or, in other words, an .vc-component of the magnetic iield that is stronger near the waveguide broad walls than at the waveguide center. FIG. 3 illustrates the static magnetic iiux lines provided by the pole piece pair illustrated as they are presently considered to exist in the waveguide.

The focusing etiect of the static magnetic eld shown in FIG. 3 will now be described by considering an electron in the outermost edge of electron stream 14 of FIG. 5. This electron occupies, in succession, positions a, b, and c between adjacent pole piece pairs. yIn position-a, the electron has the maximum undulatory displacement from axis 11 and has only a z-component, or axial component, of velocity, this component being the total velocity of the electron. It is known that a moving charged` particle ,crossing magnetic lines of llux will be subjected to a force directed perpendicul-arly to both the direction of motion of the particle and the direction of the linx, in accordance with the following vector equation:

. p =q(5 where q represents the quantity of charge of the moving particle, v represents the velocity of the particle and B represents the intensity of the magnetic lield perpendicular to the velocity vector. Thus, with the electronvelocity at position-zz directed in the z-direction and the magnetic field being oriented perpendicularly to the y-z plane, the magnetic force exerted on the electron will be toward the a-xis of the waveguide, as shown by the arrow at posi-` tion-a. When the electron reaches position-b between the next pole piece pair, the direction of the magnetic field is reversed andthe electron will be subjected to Va force again directed toward the axis of the waveguide. However, inasmuch as the static magnetic eld component encountered by the electron is greater when the electron is farther from the axis (position-a) than it is in position-b, when the electron is closer to the axis, the inward force exerted on the electron at position-a will be greater than at position-b. Therefore, the electron will be subject to a net force urging it into-the beam, due to the distribution of static magnetic flux introduced by the novel pole pieces 35 of the instant invention.

The arrow illustrated at position-c represents only the mutual electron repulsion forces on the outermost electron. The electron at position-c will also be subjected to the same inwardly directed magnetic force that it experiences at position-a. If the net inward force exerted on the outermost electron by the static magnetic field is greater than the electron repulsion force, the electron stream'will remain focused, no electrons will be intercepted by the waveguide walls, and maximum power output and efficiency will be realized from the traveling wave tube. By employing pole pieces 35 having legs dit, which provide a'stronger magnetic field near the broad waveguide walls than near the waveguide axis, such a net inward force isproduced and the electron stream remains focused.

Another form of pole piece which produces the desired variable x-component of static magnetic field between waveguide axis and broad walls is that shown in FIG. 4. In this embodiment, a triangular extension 47 is added to the end of each leg 40 to produce a variation from the magnetic pattern shown invFIG. 3. However, the embodiment of FIG. 4 also provides a stronger x-component of magnetic field near the waveguide broad walls than at the center of the waveguide. g

In another embodiment of the traveling wave tube of FIG. l, waveguide It) is circular in cross section and is illustrated as circular waveguide l in FIGS. 6 and 7. As in the embodiment of FlG. 2, the waveguide through which the electron stream and electromagnetic wave travel has disposed on opposite sides thereof a plurality of magnetic pole pieces. These pole pieces on each side of waveguide alternate in magnetic polarity, so that a static magnetic field component in the x-direction alternates with distance along the waveguide axis lli. Again, in this embodiment, the electromagnetic wave may be propagated in the transverse electric mode, The dominant transverse electric mode is the TEM mode, having the electric field distribution illustrated in FIG. 7. An electronv stream traveling along the z-axis is forced by the alternating static magnetic field to undulate in the y-z plane, the direction of the undulatory velocity coinponent being in the y-direction and parallel to the direction of orientation of the electric field component. Therefore, an energy-exchanging interaction takes place between the undulating electron streamrand the traveling electromagnetic wave, provided the conditions for synchronism described heretofore are present.

The undulation of the electron stream in waveguide lib may be described again by reference to FIG. 5, wherein the electron stream undulates about the z-axis in the y-z plane. Here, again, in the absence of the novel magnetic fields provided by the pole pieces of the instant invention, the mutual repulsion forces of the electrons will cause the stream to defocus and to be intercepted by the waveguide walls. By providing pole pieces 35 having legs 50 extending around a portion of the circumference of waveguide iti', a variable x-component of static magnetic field is provided across the height ofthe waveguide (ji-direction). This x-component of magnetic field will be stronger a substantial distance above and below axis 1I' than at axis lll', and will therefore function to focus electron stream 214 in accordance with the principles previously described in` connection with FIG. 5.

Although the invention has been described in connection with the energy-exchanging interaction involving an undulating electron stream moving with respect to an electromagnetic wave propagating in a particular transverse electric mode, an energy-exchanging interaction may also involve, and the invention is applicable to, an undulating electron stream moving with respect to a wave propagating in any other transverse electric mode Vor propagating in a transverse magnetic Inode,

Therefore, there has been described herein an invention which, by providing a novel focusing magnetic field to maintain the shape of the electron stream and to prevent interception of the stream by the waveguide walls, increases the efficiency and available power output realizable with apparatus of the type described in the aforementioned patent application.

While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many. modifications in structure, arrangement, proportions, the elements, materials, and components used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is:

ll. A charged particle stream focusing structure cornprising: means for projecting a stream of charged particles along an axis; and a magnetic field producing structure disposed along said axis forV producing a steady magnetic field perpendicular to said axis, said magnetic field alternating in direction as a function of a distance along said axis whereby said stream is caused to undulate in a direction transverse to said axis, said magnetic field producing structure including means providing a greater magnetic field strength at a predetermined transverse distance from said axis than at said axis.

2. A charged particle focusing structure comprising: an elongated hollow member; means for projecting a stream of charged particles along the axis of said member; and a magnetic field producing structure disposed along said axis for producing a steady magnetic field perpendicular to said axis, said magnetic field alternating in direction as a function of distance along said axis whereby said stream is caused to undulate in a direction transverse to said axis, said magnetic field producing structure including means providing a greater magnetic field strength at a predetermined transverse distance from said axis than at said axis.

3. Charged particle stream focusing structure'comprising: means for projecting a stream of charged particles along an axis; means for propagating an electromagnetic wave along said axis, said wave having an electric field component oriented perpendicular to said axis; magnetic means for causing said stream to undulate in a-direction parallel to said Wave electric field component, said magnetic means including a magnetic field producing structure which provides a magnetic field component perpendicular to said wave electric field and of periodically alternate magnetic polarity along said axis; and means for preventing defocusing of said stream including means for concentrating a greater amount of the magnetic iiux of said magnetic field component at theextremities of the undulatory displacement of said stream than at said axis.

d. Charged particle stream focusing structure comprising: v

an elongated hollow member;

means for projecting a stream of charged particles along the axis of said member;

means for propagating an electromagnetic wave along said axis, said wave having an electric field component oriented perpendicularly to said axis;

means for causing said stream to undulate in a direction parallel to said wave electric field component including a magnetic field producing structure disposed along said member and adapted to provide a spatially alternating magnetic field component; and means for preventing defocusing of said stream inhollow conductive tube having a circular cross section.

7. The structureV of claim wherein said electric field component is oriented parallel to one pair of opposite walls of said member, wherein said magnetic field component is oriented parallel to the other pair of opposite walls of said member and wherein said magnetic field component has a field strength greater near the walls of said other pair than at said axis.

8. A traveling wave tube comprising:

an elongated hollow member;

an electron gun disposed opposite one end of said member for projecting a stream of electrons along the axis of said member;

a transition section coupled to said member for propagating an electromagnetic wave along said axis, said wave having an electric field component oriented perpendicularly to said axis;

means for causing said stream to undulate in a direction parallel to said wave electric component including a magnetic field producing structure dis-` posed along said member for providing a magnetic field component perpendicular to said Wave electric component and periodically alternating in direction along said axis; and means for preventing the interception of said stream by said member including means for increasing the focusing force of said magnetic field on said charge particles as a function of distance of said charged particles from said axis.

9. A traveling wave tube as in claim S further including a collector disposed opposite the other end of said member for receiving the electrons transmitted therethrough.

10. A traveling wave tube as in claim 9 further including an additional transition section coupled to said member for receiving an electromagnetic wave therefrom.

11. A traveling wave tube as in claim 8 wherein said electromagnetic wave is propagated along said axis in a transverse-electric mode.

12. A traveling wave tube as in claim 11 wherein said member is a hollow conductive tube having a rectangular cross section, wherein said electric field component is oriented parallel to one pair of opposite walls of said member, wherein said magnetic field component is oriented parallel to the other pair of opposite walls of said member, and wherein said magnetic field component has a eld strength greater near the walls of said other pair than at said axis.

13. A traveling wave tube as `in claim 8 wherein said stream of electrons is projected with a velocity along said axis such that the electrons progress through a distancev substantially equal to one period of said alternating magnetic field component during a time equal to that required for the electromagnetic wave to progress through a distance substantially equalto said one period plus or minus an integral number of wavelengths of the electromagnetic eld as measured along said axis.

14. Charged particle focusing structure comprising:

means for projecting a stream of charged particles along an axis;

a plurality of magnetic pole pieces disposed along opposite sides of said axis, each one of said pole pieces being disposed between adjacent pole pieces of opposite magnetic polarity than said one pole piece, each of said pole pieces on one side of said axis being disposed facing a pole piece of opposite magnetic polarity on the other side of said axis;

and a pair of spaced-apart magnetic legs extending outwardly from each poleipiece in a direction toward the respective facing pole piece.

15. Charged particle focusing structure comprising:

an elongated hollow member;

means for projecting a stream of charged particles along the axis of said member;

a plurality of magnetic pole pieces disposed along opposite sides of said member, each one of said pole pieces being disposed between adjacent pole pieces of opposite magnetic polarity than said one pole piece, each of said pole pieces on one side of said member being disposed facing a pole piece of opposite magnetic polarity on the other side of said member;

a magnetic leg extending outwardly from each pole piece along the upper surface of said member toward the respective facing pole piece;

and a magnetic leg extending outwardly from each Vpole piece along the lower surface of said member toward the respective facing pole piece,

16. Charged particle stream focusing structure comprising:

an elongated rectangular waveguide member;

means for projecting a stream of charged particles said member, each one of said pole pieces being disposed between adjacent pole pieces of opposite magnetic polarity than said one pole piece, each of said pole pieces adjacent one of said walls being disposed facing a pole piece of opposite magnetic polarity adjacent the other one of said walls;

a magnetic leg extending outwardly from each pole piece along one of the other pair of opposite walls of said member toward the respective facing pole piece;

and a magnetic leg extending outwardly from each pole piece along the other one of said other pair of opposite walls of said member toward the respective facing pole piece.

17. The structure of claim 16 further including a transition section coupled to said member for propagating an electromagnetic Wave along said axis.

18. A traveling wave tube as in claim 17 wherein said electromagnetic wave is propagated along said axis in a transverse-magnetic mode.

References Cited by the Examiner UNTTED STATES PATENTS 2,479,084 8/ 49 Rosenthal 315-39 2,911,555 ll/59 Sensiper et al. 315-3.5 3,031,596 4/ 62 Leboutet et al. S15-3.5

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner. 

1. A CHARGED PARTICLE STREAM FOCUSING STRUCTURE COMPRISING: MEANS FOR PROJECTING A STREAM OF CHARGED PARTICLES ALONG AN AXIS; AND A MAGNETIC FIELD PRODUCING STRUCTURE DISPOSED ALONG SAID AXIS FOR PRODUCING A STEADY MAGNETIC FIELD PERPENDICULAR TO SAID AXIS, SAID MAGNETIC FIELD ALTERNATING IN DIRECTION AS A FUNCTION OF A DISTANCE ALONG SAID AXIS WHEREBY SAID STREAM IS CAUSED TO UNDULATE IN A DIRECTION TRANSVERSE TO SAID AXIS, SAID MAGNETIC FIELD PRODUCING STRUCTURE INCLUDING MEANS PROVIDING A GREATER 